A pneumonia outbreak has killed at least 33 bighorn sheep in the Mojave Desert this summer, threatening one of the state’s healthiest herds and leaving experts fearing a more widespread die-off. Wildlife managers have shot seven of the animals to test for the disease. They are considering widespread removal and killing of visibly sick animals to keep the disease from infecting other herds in the 1.6-million-acre Mojave National Preserve, which was set aside for its scenic beauty and diverse wildlife, said Debra Hughson, the preserve’s science adviser. About 300 sheep are at risk around Old Dad Mountain and Kelso Peak, 15 miles southeast of Baker, where the outbreak is centered. Pneumonia is devastating to bighorn sheep. They catch it from domestic goats, sheep and cattle, which can be unaffected by the disease. As much as 90 percent of those infected will die. The few survivors pass the disease on to lambs, with almost always fatal results, and a herd can be impacted for a decade. “To see a large-scale die-off like this, it’s a pretty hard hit to take, especially when this is a native population that has been so robust,” said a spokesman for the state Department of Fish and Wildlife Wednesday.